Oxidation catalysts based on a combination of cerium dioxide, CeO2, and metal have been reported in both non-patent and patent literature.
The work of Fu et al., published in Science (2003), describes catalytic activity of a CeO2—Au catalyst for the reaction of water with carbon monoxide, yielding carbon dioxide and hydrogen as reaction products. This is a key reaction for the production of hydrogen by hydrocarbon reforming and for the oxidation of methanol and ethanol in the direct methanol fuel cells (DMFC). In this work a CeO2—Au catalytic system in the form of a powder prepared by the traditional techniques of co-precipitation and diffusion of gold into the surface of CeO2 was used.
EP1724012A1 patent application describes a catalyst based on gold deposited on crystalline CeO2, with an average primary grain size of 5-20 nm, as well as aggregates of such crystallites 20-100 nm in size.
EP1920831A2 patent application describes a catalyst for the oxidation of solid particles in diesel engine exhaust gas, based on a combination of a mixture of two metal oxides, with Ce as one metal and Ga, Mn, Fe, Co, Ni, Cu, Yt, Zr, Mo, Ag, La, Pr, Nd, or Au as another. Catalytic material with the desired composition was prepared by precipitation from liquid solutions.
Several other patent applications deal with oxidation catalysts utilizing a combination of CeO2 with another metal: EP1676625A1, EP1683574A1, EP1787719A2, EP1889651A1, EP1852181A1, and WO2005/100249A1. In these applications, catalysts are produced by traditional procedures, e.g. by precipitation from solutions.
Some catalytic materials are prepared by sputtering. In this technique, a thin layer is deposited onto a substrate from a stream of particles (atoms, molecules) scattered from a target by energetic ions, usually argon. The work of W. D. Westwood, published in Sputter Deposition, AVS Education Committee Book Series, Vol. 2 (2003) ISBN 0-7354-0105-5, describes a whole range of sputtering techniques. In most cases, a device called magnetron is used for sputtering.
The EP0682982A2 patent application describes how an adhesion of a catalytically active metal can be enhanced by deposition of an intermediate layer of a basal metal onto a monolithic oxide substrate. Basal metal is either a metallic element from the rare earth group, or manganese. The catalytically active noble metal (platinum in this case) is deposited on top of this layer.
Amongst the main drawbacks of the catalysts mentioned above is either too low catalytic activity, or too high content of noble metals, mainly gold and platinum. Furthermore, powder catalysts containing CeO2 and a noble metal suffer from too low dispersion of the active metal in CeO2. Other common drawback of the aforementioned catalysts is low resistance against poisoning.
The techniques for preparation of metal-CeO2 catalysts known so far do not allow the production of catalysts active enough to be used in, for instance, oxidation of methanol or ethanol in direct fuel cells.
Excessive use of noble metals leads to the increase of production cost of such a catalyst. Mass production of catalysts with high content of noble metals also burdens the environment.